The ubiquitous P(o-tol)3 ligand promotes formation of catalytically-active higher order palladacyclic clusters

The Herrmann-Beller catalyst, Pd[(C^P)(μ2-OAc)]2, is readily formed by reaction of the cyclic trimer of ‘Pd(OAc)2’ with P(o-tol)3. In the presence of hydroxide, Pd(C^P)(μ2-OAc)]2 converts to [Pd(C^P)(μ-OH)]2. Here, we report how this activated Pd precatalyst species, and related species, serve as a conduit for formation of higher order Pdn clusters containing multiple cyclopalladated P(o-tol)3 ligands. The catalytic competency of a Pd4-palladacyclic cluster is demonstrated in an arylated Heck cross-coupling, which is comparable to the base-activated form of Herrmann's catalyst, namely [Pd(C^P)(μ2-OH)]2. The findings show that ‘simple’ ubiquitous phosphine ligands can promote higher order Pd speciation, moving beyond well-known phosphine-ligated Pd1 and Pd2 complexes. The findings challenge the status quo in the field, in that phosphine ligands can ligate higher order Pdn species which are catalytically competent species in cross-coupling reactions.